A strain gauge is a strain-sensitive device employed to sense strain, such as that caused by stress in the form of tensile or compressive forces applied to a structure. Conventional strain gauges typically employ a strain sensing element adhered to at least one surface on or within the structure such that, when the structure exhibits a strain in response to an applied stress, the resistance of the sensing element changes in proportion to the sensed strain. The measured strain is generally calculated based on the change in resistance in the sensing element as the structure is compressed or elongated, thus exhibiting or manifesting the strain. Strain gauges can be used to measure bending, axial and torsional strain, or a combination of strain effects, on a structure resulting from various applied loads.
Strain gauges may include foil-type strain gauges comprising a pattern of resistive foil mounted on a backing surface. Furthermore, strain gauges may include semiconductor strain gauges, which are often preferred over foil gauges when measuring small amounts of strain. Strain gauges may be attached to a flexible plastic substrate that, in turn, is bonded to the structure for which the strain is to be determined.
A sensing element for a strain gauge is conventionally implemented within a Wheatstone bridge circuit, which converts the sensed resistance to a voltage signal. To obtain the voltage signal, it is generally required to further connect a differential amplifier and a current source to the Wheatstone bridge circuit. FIG. 1 is a schematic diagram of Wheatstone bridge circuit 100 including four branches, one of which may include a resistive transducer, such as a strain gauge 110. The other branches of Wheatstone bridge circuit 100 include resistors R1, R2, and R3. An input DC voltage, or excitation voltage Vin, is applied between the top and bottom of circuit 100 and an output voltage Vout is measured across the middle of circuit 100. When the output voltage is zero, circuit 100 is balanced. As the resistance of one of the branches changes, by a strain of a resistive strain gauge for example, the previously balanced circuit becomes unbalanced. This unbalance causes a voltage Vout to appear across the middle of circuit 100. This induced voltage may be measured with a voltmeter or the resistor R3 in the opposite branch may be adjusted to rebalance circuit 100. In either case, the change in resistance that caused the induced voltage may be measured and converted to obtain a degree of strain.
FIG. 2 illustrates a conventional strain sensing system 200. System 200 includes a strain sensor 210 in electrical communication with a sensing system 220. Strain sensor 210 may include circuitry, such as the Wheatstone bridge circuit 100 shown in FIG. 1. Furthermore, strain sensor 210 may be coupled to a structure for sensing strain responsive to stress experienced by the structure due to an applied force or forces. Strain sensor 210 produces an electrical signal that is used by the sensing system 220 for identifying the strain force on the object and presenting the identified force to an observer.
The accuracy of data reported by a sensor, such as a strain gauge, mounted to a structure depends to a high degree on the integrity of the bond, such as an adhesive bond, between the sensor and the structure to which it is secured. It is generally accepted that the adhesive bond (e.g., an epoxy) may break down or debond over time due to various conditions, such as by way of non-limiting example, fatigue, corrosion or bond degradation due to exposure to elements, such as moisture, subjection to temperature extremes, or simply long-term degradation of the adhesive bond over an extended period of time. A sensor which is debonded from a structure even slightly may result in failure or malfunction of the sensor manifested as a complete lack of sensor output and may, at best, provide incorrect strain measurements in the form of a reduced-magnitude or otherwise misleading output. Furthermore, conventional sensors do not always provide sufficient, if any, warning of potential failure or malfunction due to partial or complete disbonding.
The bond between a sensor and a structure of interest to which the sensor is secured is conventionally inspected by visual or tactile means, which are time consuming and often inconclusive. Furthermore, a sensor may reside in a location not accessible to a human inspector, such as within an interior of a structure wall or within a sealed compartment.
There is a need to enhance the efficiency and reliability of measuring a physical property on a structure of interest. Specifically, there is a need for methods, devices, and systems for verifying the integrity of a bond between a sensor for measuring a physical property of a structure and the structure of interest.